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Tesla Coil Build Thread

Sigurthr

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So, I finished Steve Ward's SSTC-6 "Micro SSTC" today.

I blew up a fet too. Thermal destruction. Man, these things put out a ton of heat. Good thing I ordered 2 spares. It is running continuously at 2/3 power right now. I need to solve the thermal issue though as the heatsinking I'm using on it now is not practical.

I am disappointed in the output though. I know I'm only running it on 24V instead of 36V but still, no corona or streamers at breakout what-so-ever. I followed Steve Ward's schematic to a "T" with the exact parts as well. It doesn't like to start oscillating unless I bypass the dc blocking capacitor from the 555 timer either, which he didn't have an issue with. He did say the resistor string and dc blocking cap were only meant to weaken the 555's signal so that the feedback from the antenna overrides it, so bypassing the cap doesn't do any real harm, but it is annoying to have to pushbutton start it.

photo2wwc.jpg


photo1xro.jpg


I desperately need to find a better form of current limitation for it as well. It wants to draw a little over 8 AMPS, yet Steve's design specifies a rectified 25VAC (~36VDC) 2A transformer as the power supply. I'm using beefy wirewound resistors right now and they are overheating on long run times while only letting 2.5A through at only 24VDC. I'm thinking of increasing the primary turns as this would increase the impedance and limit current, but it would also drop the output voltage as the turns ratio would be reduced. It would help with the thermal issue though so it may be worth doing anyway.

Ideally I want this to be a table top unit capable of being left on for hours. If the fet still runs real hot with the increased primary impedance I'll try adding a fan to cool the heatsink. After that all I can think of is to add a topload (which Steve says not to) to lower the resonant frequency which should lower switching losses.

Any input?
 



Hiemal

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So, I finished Steve Ward's SSTC-6 "Micro SSTC" today.

I blew up a fet too. Thermal destruction. Man, these things put out a ton of heat. Good thing I ordered 2 spares. It is running continuously at 2/3 power right now. I need to solve the thermal issue though as the heatsinking I'm using on it now is not practical.

I am disappointed in the output though. I know I'm only running it on 24V instead of 36V but still, no corona or streamers at breakout what-so-ever. I followed Steve Ward's schematic to a "T" with the exact parts as well. It doesn't like to start oscillating unless I bypass the dc blocking capacitor from the 555 timer either, which he didn't have an issue with. He did say the resistor string and dc blocking cap were only meant to weaken the 555's signal so that the feedback from the antenna overrides it, so bypassing the cap doesn't do any real harm, but it is annoying to have to pushbutton start it.

photo2wwc.jpg


photo1xro.jpg


I desperately need to find a better form of current limitation for it as well. It wants to draw a little over 8 AMPS, yet Steve's design specifies a rectified 25VAC (~36VDC) 2A transformer as the power supply. I'm using beefy wirewound resistors right now and they are overheating on long run times while only letting 2.5A through at only 24VDC. I'm thinking of increasing the primary turns as this would increase the impedance and limit current, but it would also drop the output voltage as the turns ratio would be reduced. It would help with the thermal issue though so it may be worth doing anyway.

Ideally I want this to be a table top unit capable of being left on for hours. If the fet still runs real hot with the increased primary impedance I'll try adding a fan to cool the heatsink. After that all I can think of is to add a topload (which Steve says not to) to lower the resonant frequency which should lower switching losses.

Any input?

Jeez, that poor mosfet must have been really really hot.

MOSFET's can typically take some abuse in terms of temperature..... if that's any indication.

Could you show me the schematic you're using for the coil? Using just one mosfet kind of sucks. Half bridges are the way to go!
 

Sigurthr

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Jeez, that poor mosfet must have been really really hot.

MOSFET's can typically take some abuse in terms of temperature..... if that's any indication.

Could you show me the schematic you're using for the coil? Using just one mosfet kind of sucks. Half bridges are the way to go!

Yes the first mosfet reached over 200C in about seven seconds while thermal epoxied to a 40gm piece of 1/4" thick copper. It caught me off guard, but it was switching nearly 9A @ 24V (~215W) through 0.25Ohms of RdsON resistance. I did get a small streamer from that setup before the fet died.

With the new fet in I'm limiting current with 4 10W wirewound resistors in parallel and they are glowing incandescently after a long run. No streamers at all, but current is limited to only 2.5A instead of the >8A it wants to pull.

Here is the schematic I used: (the second one with the 555 for auto-start oscillator) The Micro SSTC

I've followed everything perfectly, so I don't know what is wrong. I doubt having 10VDC less is making that huge of a difference and I'm not sure how the 2A transformer will handle a >8A load... talk about potential for voltage sag and saturation. Still doesn't explain why my fet is so damn hot either. He is using a smaller sink than I am!

Also; I'd love to move up to a rectified mains half bridge. I've never built a half bridge, or a full bridge, for power amplification/inverter before. I've built them for motor controll but there was NO oscillation there, it was just to switch polarity to a load via a toggle switch. I've never wound a GDT either, even though I know the theory behind it... trifillar winding.. one wind is from the oscillator, one wind to 1st gate, 2nd wind reverse phase to second gate. It seems everyone is using IGBTs for that stuff though and I have no experience with them so I don't really know what ones would be suitable. Plus I don't have a scope so trying to make sure I achieve ZCS would be an utter nightmare.

Update:
I was able to rig up a blower to the heatsink + copper plate to see how hot it would get without the resistors and to also see the voltage sag under the huge load. I used my good DMM's temperature probe to monitor temperate at the junction of the fet to the aluminium heatsink. I stopped the test after a few minutes when the temp hit 80C (176F). No noticeable increase in output voltage from the secondary during the test either. Voltage sagged to 14V under the heavy 9amp load, so this probably had something to do with the decrease in output I actually noticed, as it was getting 24V @ 2.5A before and 14V @ 9A now.

R = V/I so that would mean the fet has an RdsON resistance of 1.55 ohms. Does this confirm my theory of the fet operating in the linear region and thus incurring huge losses?
 
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Seoul_lasers

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Sounds like you might have the MOSFET wired improperly. We won't know until you take a better picture of your circuit and upload the schematic from what you want to work from.
MOSFETS do get hot, but what you are explaining does not sound normal.
 

Hiemal

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It does sound like something is amiss, but in a single mosfet Tesla coil driver it really doesn't surprise me. After all, one switch has to handle ALL of the load...

There should be other SSTC drivers out there that use half bridge setups... I'll try looking around to see what there is.

What frequency is your SSTC running at?

As I said, single mosfet drivers suck. A lot. But, based off of your schematic, it shouldn't be getting THAT hot THAT quickly.
 
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Sigurthr

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Sounds like you might have the MOSFET wired improperly. We won't know until you take a better picture of your circuit and upload the schematic from what you want to work from.
MOSFETS do get hot, but what you are explaining does not sound normal.

What you see in Steve's schematic is EXACTLY how everything is wired. There are no variations and no errors. I've quadruple checked everything.

Just to note, I asked on 4HV about the lack of breakout and someone said they built a similar one recently (but using lower RdsON IRFP260s instead of the 460s) and had to add more primary turns and raise the coupling heavily to achieve breakout.

I changed the primary to 10 turns and raised it to center of the secondary and got continuous 3/8" streamers. Current draw dropped to 1 to 2.5 amps. I adjusted the antenna for optimum streamer length and was able to reach 1/2" streamers but the current draw rose to 3.25A.

I had the fet thermal epoxied to a large aluminium and copper heatsink which was clamped down to a 1'x1'x1/8" copper plate and a hairdryer fan on full (no heater elements) blowing on the Al/Cu heatsink.

It was running at 30-40C with the current draw at ~2A, but when I adjusted the antenna and the current draw rose to over 3A the fet quickly got much hotter and peaked at 90C before dying. I'm guessing the max temp listed on the data sheet is the junction temperature and NOT the tab/case temperature. 80C seems to be the maximum case/tab temp these can handle.

Only one IRFP460 left.... debating ordering some IRFP260s and heatsinks.....


@ Jared, aye that's his finished version of the Mini SSTC. I think I may make that next but the parts come to over $30 for just the power ICs/transistors. I havent tabulated everything up yet but it looks like it would cost around $60 in parts, if not more. I'm completely out of budget right now, was really hoping this micro sstc would work well.
 

Polonium210

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I also made the micro SSTC but instead of using the IRFP460's I used the 260's and had great results. I used a heatsink I found is some electronic junk, I think an old TV :thinking:
The heatsink gets a little warm but it's expected with the amount of current I'm allowing the coil with the beefy 24v transformer.

Here's a video I made showing the coil and a "Glow Rod" I made.

I get my toroids from John Freau -> futuret@aol.com :D
 
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Seoul_lasers

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The spark length on your TC is limited to 1-1.25" maximum.
Good to hear the sparks occurring. Video? so it's a tuning issue... I see.

btw,no offense just a little point.... what is this "aye" in your posts, are you trying to fein Scottish ancestry ? I was wondering if you were doing a "James Doohan special or somthing"
 
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Sigurthr

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Haha, no offense taken. Geez if my posts sound like James Doohan in your head that is hilarious. I've said "aye" since I was about 4 years old. My family has no idea where I picked it up. It's quite persistant, and as I type what I would say if I were speaking to someone, it comes out in text as well. Although, some places I do redact it... official forms, interviews, and such it doesn't usually go over so well. I do get unusual looks sometimes when meeting someone new.

Somewhat related... I also say "Da" every now and then. I usually don't notice it when it happens until someone else points it out to me, usually via "what the hell was that?". I don't speak any russian though. The only foreign words I purposely inject in to speech are german and italian ones since I used to be fluent in both when I was younger, and they never come out in text.

Back to TCing....

@Po210 Wow that looks great! That is the same schematic I'm using? Those results are infinitely better that anything I've seen so far. How much current are you feeding it? Also, it's good to know that John still makes toroids! If you don't mind me askin, how much did that toroid run ya? (pm maybe?).

So it looks like IRFP260s are the way to go.... I'll have to order some asap.
 
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Polonium210

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@Po210 Wow that looks great! That is the same schematic I'm using? Those results are infinitely better that anything I've seen so far. How much current are you feeding it? Also, it's good to know that John still makes toroids! If you don't mind me askin, how much did that toroid run ya? (pm maybe?).

So it looks like IRFP260s are the way to go.... I'll have to order some asap.

It is the same schematic you are using. I think it draws around 4-5 amps.

A few things to try since your secondary is of different dimentions than mine is to try different capacitance values for C3 in the schematic (the cap that's across drain and source of the Fet) along with more or less primary turns. This will find the sweet spot on your coil and bring it close to if not into the "Class E" category. Try series or paralleling two 1nF 2Kv caps and see if you get better results less stress (heating of the Fet).

I'm not sure on Johns current prices for his toroids and I do not recall how much this one was, I want to say $30 maybe? But definately worth every cent with the larger ones!!!
For your single Fet coil try using a hard drive platter as a top load.
 

Sigurthr

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I made a video: "Micro" Solid State Tesla Coil (SSTC) - V1.0 - YouTube


Someone on 4HV suggested I add protection diodes. I am all out, I tried a 1N60 but it blew instantly (but the fet was fine). I then put on the only diode I had left... 1N4003. It runs!

Now, I'm still learning these more advanced electronics things... can anyone please explain to me why adding a slow reverse biased diode in parallel with the mosfet's drain/source would reduce power consumption 600%? The coil only draws 600mA without any current limiting resistors and the Fet runs cool as can be continuously.

There are two 1/8" baby streamers squeeking away now.

Also, I just placed an order for a bunch of IRFP260s, BEEFY TO-247 heatsinks, and MUR860 protection diodes, so I'll have replacement parts on hand for if this fet dies. I'm over budget but sitting there for an hour with the coil dead depressed me so much I couldn't stand it.

Also, I rebuilt the spark gap on my SGTC with a hardryer fan for much better quenching. Will test it tomorrow!


UPDATE: 5/10/12
I rebuilt the SSTC last night and achieved a nice happy medium between output and waste heat. The coil runs continuously now and output of up to 15mm is present when a capacitively coupled load is near the output.

Here's a new, better video:
http://www.youtube.com/watch?v=PSrrzm_LGz0
 
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Seoul_lasers

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interesting how the corona increases when a grounded object is placed within the vicinity of the coil. Curious.. have you tried using a larger top load on the coil yet? I have a feeling your output corona will get larger. You'd be lowering your resonant frequency.
Coke can?
 

Sigurthr

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That was my assumption as well, unfortunately it behaves oppositely! I tried a small ball of aluminium foil with the breakout point attached to it, and there was no more corona/streamer at all. I tried a Coke can and the E field seemed to diminish in intensity and range even more. I wonder if the can was interfering with the magnetics... acting as a shorted turn... perhaps I should try a long lead from top terminal to the topload so as to isolate it from the m-field. Not sure how I would implement that though.
 
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Seoul_lasers

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What about increasing the amount of primary turns and then adding a larger top load?
 

Sigurthr

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Hmm, that's something I haven't thought to try. I'm expecting my 4" sphere tomorrow (for my SGTC) and I can give it a shot with that. I'll have to use wire nuts and some spare lengths of the good wire I have left as I finishing my spool the other day. I still have two sections about 3 feet long, which should be enough for a couple more turns.

I'm all out of white electrical tape though, so I'll have to pick some up. I was thinking of making another small secondary anyway so it's needed regardless. (I don't like to put black tape right on the secondary winding, it's ok for over the white stuff though).

More turns sounds appealing to me as it would increase primary impedance and reduce current draw which reduces mosfet heating. I'd like to try and make a nice full H bridge with the four IRFP260s I recently ordered.



UPDATE: I moved the coil to a more stable wood table and proceeded to try more turns on the primary. I fired her up at 11 turns and she promptly shut off. I check the RdsOFF resistance and it reads 0.1ohms. I know what this means =/. Rgd is 5ohms, another dead fet. I removed it from the heatsink and circuit and noticed something odd this time...

deadfet.jpg


Is it just me or does that NOT look like electrical damage? Did the pad on the back actually start to melt!?!?! I used Ceramique2 thermal compound and this was on a fan cooled heatsink!

IRFP460s are forever dead to me now.

I should have my IRFP260s, MUR860s, and TO-247 finned heatsinks in the mail tomorrow or early next week. I was kind of planning on making a full H bridge to run off rectified mains with them though. Maybe I will, maybe it is time to scrap this driver. 3 fets dead and it's a strike, right? There's only about $15 worth of parts on the driver board, and everything except the 555 and TC4420 is reusable (and I have spares of those).

I'd like someone experienced with H bridges and GDT's to look over my design if possible. (Jared, you out there?)
 
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